The present invention relates to a method for manufacturing an acoustic surface wave interaction device.
An acoustic surface wave interaction device generally includes a plate of piezoelectric material and a set of interdigitated electrodes laminated on one flat surface of the piezoelectric plate. The piezoelectric plate is polarized between said one surface and the other surface to align the molecules contained in it parallel to each other. According to the conventional method, the acoustic surface wave interaction device is manufactured by the following steps.
First, the plate of piezoelectric material is laminated with a film of conductive material on its opposite flat surfaces. DC voltage is then applied between the two conductive films to polarize the piezoelectric plate. The film of conductive material on one surface is partly removed to form interdigitated electrodes in any desired arrangement by means of any known method, e.g. photolithography. To prevent deterioration of the electrode on exposure to air, and also for the purpose of improving the effective electro-mechanical coupling factor Keff, the side of the piezoelectric plate on which the interdigitated electrodes are formed is coated with a thin layer of dielectric material.
This thin layer of dielectric material is normally formed by a sputtering or vacuum evaporation method. In this method, the temperature of the device is increased during the process of formation of the thin layer of dielectric material so that the thin layer can rigidly be connected to the piezoelectric plate. However, such increased temperature tends to cause the deterioration of the polarization of the piezoelectric plate to such an extent that not only is the coupling factor Keff reduced, but also the insertion loss increases. (It is to be noted that the term "insertion loss" above and hereinafter is intended to mean an energy loss which takes place in the surface wave interaction device when electrically connected to a circuit.) On the other hand, if the thin layer of dielectric material is formed at a low temperature, there will hardly be any deterioration of the polarization of the piezoelectric plate, but the thin layer of the dielectric material will not be rigidly connected to the piezoelectric plate and, therefore, after a period of time, the thin layer can easily separate from the piezoelectric plate.
Furthermore, during the step of polarization of the piezoelectric plate, the surface of the film of the conductive material may be undesirably stained with oil. Accordingly, in the subsequent step of photoetching for removing a part of the film of the conductive material to form the electrodes, the oil on the film tends to hamper complete removal of that part of the film, having a width of several tens of microns, between the electrodes with the result that the neighbouring fingers of the interdigitated electrodes may not be completely separated from each other.